The present invention relates to a semiconductor light emitting device for use in optical communication equipment, information display panels, CCD (Charge Coupled Device) camera auxiliary light sources, LCD (Liquid Crystal Display) backlights, lighting equipment and the like.
In recent years, LEDs (Light Emitting Diodes) are widely used in optical communication equipment, information display panels, CCD camera auxiliary light sources, LCD backlights, lighting equipment and the like. High luminance is important for the LEDs for use in these applications. Particularly in these days, use of the LEDs as lighting equipment has started, which increases the needs for high luminance. One method for achieving high luminance is to enhance efficiency of LEDs. The efficiency of the LEDs is determined by internal quantum efficiency and external emitting efficiency, among which the external emitting efficiency is largely influenced by device structure.
One example of the method for enhancing the external emitting efficiency of the LED is to use substrates transparent against emitting wavelength. In the case of AlGaInP (aluminum gallium indium phosphide)-based LED, conventional manufacturing methods for the LED having a substrate transparent to emission wavelength include one shown in FIGS. 9A, 9B, 9C and 9D (see, e.g., JP H06-302857 A). In this manufacturing method for LED, on a GaAs (gallium arsenide) substrate 101 opaque to emission wavelength, an AlGaInP-based emission layer 102 is epitaxially grown, and a GaP current diffusion layer 103 is grown thereon for several dozen μm thickness (see FIG. 9A). Next, the GaAs substrate 101 opaque to emission wavelength is removed (see FIG. 9B), and the GaP substrate 105 is joined to the removed face by heat treatment (see FIG. 9C). As shown in FIG. 9D, a substrate-side electrode 106 and a top-side electrode 107 are formed.
In the manufacturing method for the LED, the AlGaInP-based emission layer 102 is not matched to GaP, and therefore once the emission layer 102 is grown on the GaAs substrate 101, the GaAs substrate 101 is removed and the GaP substrate is attached thereto. The GaP current diffusion layer 103 grown on the emission layer 102 is set to have a size of 50 to 100 μm in view of a growth time and a mechanical strength of wafers after removal of the GaAs substrate 101. If the layer thickness of the GaP current diffusion layer 103 is 40 μm or less, the wafer is extremely fragile during handling, whereas if the layer thickness is 100 μm or more, the growth time is prolonged and the cost of the LED becomes high.
However, even if the layer thickness of the GaP current diffusion layer 103 is set at 50 to 100 μm, the mechanical strength is still not enough and the problem that wafers break during handling is not yet solved.
Accordingly, in order to solve the problem, there is another manufacturing method for AlGaInP-based LED (see JP Patent No. 3230638) as shown in FIGS. 10A, 10B, 10C and 10D. In this LED manufacturing method, on a GaAs substrate 111 opaque to emission wavelength, an AlGaInP-based emission layer 112 is epitaxially grown, and a GaP layer 113 is formed thereon (see FIG. 10A). Then, on the GaP layer 113, a GaP substrate 115 transparent to emission wavelength is placed, and heat treatment is applied thereto for establishing joint (see FIG. 10B). Then, the GaAs substrate 111 opaque to emission wavelength is removed (see FIG. 10C). As shown in FIG. 10D, a substrate-side electrode 116 and a top-side electrode 117 are formed.
In this LED manufacturing method, it is not necessary to handle wafers with a thickness of 50 to 100 μm, and therefore the problem that the wafers break during manufacturing process can be solved.
Another method for achieving high luminance in the LED is to increase a current injected into the LED.
Problems arising from increase in a current injected into the LED include heat saturation caused by heat generation in the emission layer. With increase of the injection current, a heat quantity generated in the emission layer exceeds a heat quantity radiated outside mainly through a die bonding portion, which increases a temperature of the emission layer, resulting in overflow of carriers and saturation of optical output.
In order to solve the problem, an AlGaInP-based LED as shown in FIG. 11 has conventionally been proposed (see Nikkei Electronics by Nikkei Business Publications, Inc., Oct. 21, 2002). In this LED, an Si (silicon) substrate 121 with thermal conductivity three times higher than conventionally-used GaAs substrates is attached to a semiconductor layer 123, which is connected to an emission layer 124, through a metal layer 122. In this LED, the emission layer 124 is epitaxially grown on the GaAs substrate, and after the Si substrate 121 is attached thereto, the GaAs substrate is removed. In FIG. 11, there are shown a substrate-side electrode 126, a top-side electrode 127 and a current block layer 128.
The LED is free from large heat resistance from the GaAs substrate, which makes it possible to considerably reduce a loss of heat radiation.
However, the conventional LEDs shown in FIGS. 9D, 10D and 11 suffer the problem that luminance is degraded due to other causes.
More particularly, AlGaInP-based semiconductors have a refraction index of about 3 to 3.5, whereas air has a refraction index of 1 and resin has a refraction index of about 1.5. Therefore, there are differences in refraction index of 2 to 2.5 and 1.5 to 2 between air and an AlGaInP-based semiconductor layer, and between resin and the AlGaInP-based semiconductor layer, respectively. With the differences in refraction index, among light beams generated in the chip, the light beams incident to the surface of an LED chip at angles not less than a critical angle are totally reflected. The critical angle is approx. 17° in the case where light beams come incident from the AlGaInP-based semiconductor layer to air, while the critical angle is approx. 25° in the case where light beams come incident from the AlGaInP-based semiconductor layer to resin.
Therefore, in the conventional LEDs shown in FIGS. 9D and 10D in which the GaP substrates 105, 115 are joined to the GaP layer 113 or the emission layer 102, the chips are in an almost rectangular parallelopiped shape, and so a proportion of light beams totally reflected on the chip surface to light beams generated in an active layer is relatively high. The totally reflected light beam is, as schematically shown in FIG. 12, multiple-reflected in the chip, passes an active layer 131 a plurality of times, and is reflected by a plurality of electrodes 132. In this case, the multiple-reflected light beam is absorbed by the active layer 131 and the electrodes 132. In FIG. 12, the positions at which the multiple-reflected light beam is absorbed by the active layer 131 and the electrodes 132 are shown by symbol A. Thus, as a result of absorption of the light beam at a plurality of positions, the luminance of the LED is degraded.
Moreover, in the conventional LED with the Si substrate 121 attached thereto in FIG. 11, the semiconductor layer 123 transparent to emitting light, the emission layer 124 and the semiconductor layer 125 are relatively thin with respect to the entire LED, which makes the proportion of multiple-reflected light beams larger than that in the LEDs in FIGS. 9D and 10D. Therefore, the number of times that a light beam generated in the active layer passes the active layer and the number of times that the light beam is reflected by the electrodes further increase, which in turn increases the absorbed quantity in the active layer and the electrodes and causes large degradation in luminance of the LEDs.